1. Field of the Invention
The present invention is directed toward endless fabrics, and more particularly, fabrics used as industrial process fabrics in the production of, among other things, wetlaid products such as paper, paper board, and sanitary tissue and towel products; in the production of wetlaid and drylaid pulp; in processes related to papermaking such as those using sludge filters, and chemiwashers; in the production of tissue and towel products made by through-air drying processes; and in the production of nonwovens produced by hydroentangling (wet process), meltblowing, spunbonding, and airlaid needle punching. The term “industrial process fabrics” also includes but is not limited to all other paper machine fabrics (forming, pressing and dryer fabrics) for transporting the pulp slurry through all stages of the papermaking process.
2. Description of the Prior Art
During the papermaking process, a cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, onto a moving forming fabric in the forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric.
The newly formed cellulosic fibrous web proceeds from the forming section to a press section, which includes a series of press nips. The cellulosic fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two such press fabrics. In the press nips, the cellulosic fibrous web is subjected to compressive forces which squeeze water therefrom, and which cause the cellulosic fibers in the web to adhere to one another to turn the cellulosic fibrous web into a paper sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the paper sheet.
The paper sheet finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The newly formed paper sheet is directed in a serpentine path sequentially around each in the series of drums by a dryer fabric, which holds the paper sheet closely against the surfaces of the drums. The heated drums reduce the water content of the paper sheet to a desirable level through evaporation.
It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speed. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
The present invention primarily concerns the papermaking fabrics which run on the various sections of a paper machine, as well as to fabrics used in other industrial settings where fabric surface smoothness, fiber support, non-marking, planarity and controlled permeabilities to water and air are of importance. Examples of the papermaking fabrics to which the invention applies are forming fabrics which run in the forming section of a paper machine, press fabrics which run in the press section, and drying fabrics which run in the drying section. Another example of an industrial process fabric to which the invention can be applied is a through-air-drying (TAD) fabric. A TAD fabric can be used in a variety of industrial settings, including papermaking. Some fabrics can be processed to act as a transfer fabric and can either be permeable or impermeable.
Papermaking fabrics, especially forming and drying fabrics, are generally woven in flat form and joined into endless-loop form with a seam. During the weaving process, the warp yarns, generally plastic monofilaments, are interwoven with weft, or filling yarns, also generally polymeric plastic monofilaments, in a desired pattern. In a fabric woven in flat form, the warp yarns eventually lie in the machine, or running direction of the fabric, while the weft yarns lie in the crossmachine direction.
After weaving, the fabric is heatset. The heatsetting, in which the fabric is placed under tension in the warpwise direction in the presence of heat, transfers some of the warp crimp to the weft yarns, smoothing the surface of the fabric to a degree and stretching the fabric in the warpwise direction to reduce the amount it could possibly stretch during use on a paper machine. Seaming or joining techniques are then employed to process the fabric into an endless loop as known in the art. For endless woven or modified endless woven fabrics, the processes form a complete tube of approximately the required length and width. Modified endless weaving results in a seam to allow easy installation on the machine. The weft yarns are now the MD yarns, and the warp yarns are CD yarns. The fabric is also heatset for sizing and crimp transfer and batt fiber is subsequently applied to one or both surfaces by processes such as needling.
As part of the later or last manufacturing steps, the surface of the fabric may be further smoothed by grinding, or sanding, which reduces the difference in height between the knuckles formed by the warp yarns and those formed by the weft yarns. Unfortunately, the grinding is essentially a form of wear which occurs before the fabric is even shipped to a customer, and potentially reduces the useful life span of the fabric.
In the case of press fabrics, the fabric can be pre-compacted under heat and pressure to cause some densification of the fabric by reducing thickness. This does not cause permanent fiber deformation.
Finally, the heatset, possibly needled and possibly ground, endless fabric loop of desired length and width is shipped to a customer for installation on the forming, press or dryer section of a paper machine, or use on a nonwovens machine.